The present invention relates to a method for treating (phosphating) a metal surface with zinc phosphate being provided for coating etc. and, in detail, to a phosphating method for forming a zinc phosphate coating film on an iron-based, zinc-based, and an aluminum-based surfaces as well as a metal surface having these two or more surfaces in combination and simultaneously, wherein the coating film is desired to be suitable for electrocoating, in particular, for electrocoating of a cation type and to be superior in the adhesion character, corrosion-resistance, in particular, in warm brine-resistance and resistance for rust of a scab type (hereinafter, referred to as scab-resistance).
Metal materials have been used in various fields such as automobile bodies and other automobile parts, building materials, furniture and so on. The metals are treated with zinc phosphate (phosphating) as a coating pre-treatment in order to prevent corrosion due to oxygen, sulfur oxides in the air, rain water, and sea water etc. The zinc phosphate coating film thus-formed requires adhesion-superiority with a metal surface that is a substrate and with a coating film thereon formed (secondary adhesion) as well as to have sufficient rust-resistance even under corrosive surroundings. In particular, since the automobile body repeatedly suffers contact of brine and variation of weather conditions (dry or wet) at a wounded place of the outside plate, it is desired to have scab-resistance and a higher degree of warm brine-resistance.
Recently, there are increasing the cases of treating metal materials composed of two or more kinds of metal surfaces with zinc phosphate. For example, in order to further elevate corrosion-resistance of the automobile body after the coating, a zinc- or zinc alloy-plated material is used in only one side of steel materials. Like this, when the hitherto known phosphating treatment with zinc phosphate is carried out for a metal surface having both an iron-based and zinc-based surfaces, there is caused a problem that the corrosion-resistance and secondary adhesion on the zinc-based surface is inferior compared with those on the iron-based surface. Because of this, there has been proposed, for example, in Japanese Official Patent Provisional Publication, showa 57-152472 etc., a method of forming a zinc phosphate coating film which is suitable for electrocoating on a metal surface having both an iron-based and zinc-based surfaces simultaneously. In this method, manganese ions in a concentration of 0.6.about.3 g/l and/or nickel ions a concentration of 0.1.about.4 g/l are contained in a treating bath wherein the concentrations of zinc ions, phosphate ions, and a coating film-converting accelerator are controlled. Also, there has been proposed in Japanese Official Patent Gazette, showa 61-36588, an art wherein fluorine ions are added in a concentration of 0.05 g/l or more, together with manganese ions in order to lower treating temperature.
Also, a material composed of an aluminum material combined with an iron or zinc material has practically been used in various fields such as the automobile and building materials etc. When the kinds of materials are treated with an acidic, treating (phosphating) solution for forming a zinc phosphate coating film, aluminum ions dissolving into the treating solution is accumulated and, if its amount increases to a certain extent, there is a problem of inferior conversion which takes place on an iron-based surface. That is, if the aluminum ions increase up to a concentration of 5 ppm or more in a treating solution not containing the fluoro ion, to a concentration of 100 ppm or more in a treating solution containing HBF.sub.4, and to a concentration of 300 ppm or more even in a treating bath containing H.sub.2 SiF.sub.6, there has been found conversion inferiority on an iron-based surface.
Thus, in order to prevent the increase of aluminum ions in a treating solution, there has been proposed in Japanese Official Patent Provisional Publication, showa 57-0281, a method wherein the aluminum ions are precipitated as a form of K.sub.2 NaAlF.sub.6 or Na.sub.3 AlF.sub.6 adding acidic potassium fluoride or acidic sodium fluoride to the treating solution. Also, there has been proposed in Japanese Official Patent Provisional Publication, showa 61-104089, a method wherein proportion of an aluminum-based surface area to an iron-based surface area is controlled to 3/7 or less and the aluminum ion concentration in a treating solution of fluorine-based zinc phosphate is maintained at 70 ppm or less.
On the other hand, a method of forming a zinc phosphate coating film on an aluminum-based surface and being provided for cationic electrocoating has been proposed, for example, in Japanese Official Patent Provisional Publications, showa 63-157879 and 64-68481. In the former publication, there has been disclosed a method wherein a metal surface is brought in contact with a treating solution for forming zinc phosphate coating film which contains a fluoride [F (el)], when measured with a fluorine ion-sensitive electrode, in a concentration of 80.about.220 mg/l and an acidity of the free acid is adjusted in proportion to the F (el) concentration. In the latter publication, there has been disclosed a method wherein a metal is brought in contact with an aqueous treating solution for forming zinc phosphate coating film containing said F (el) in a concentration of 80.about.400 mg/l and proportion of the free acid to the total acid is adjusted in a ratio of (0.02.about.0.15):1.
The method for treating with the zinc phosphate which was described in Japanese Official Patent Provisional Publication, showa 61-104089, has a disadvantage so that a treating object is very limited and also, it is difficult to maintain the aluminum ion in a concentration of 70 ppm or less by only controlling the area proportion as described above. In contrast, the treating method which was described in Japanese Official Patent Provisional Publication, showa 57-70281, is superior in a point of view that it does not limit a treating object, but remove aluminum ions from a treating solution with precipitating. However, the precipitate here formed shows a trend of floating and suspending and attaches to a zinc phosphate coating film causing ununiformity. Because of this, in a case where an electrocoating is carried out on a zinc phosphate coating film, an inferior electrocoating takes place which becomes an origin for causing lack of uniformity and bad secondary adhesion of a coating film etc. Thus, it is necessary to remove the precipitate of floating and suspending character, but this removing process is complicate.
Also, in the methods for treating with the zinc phosphate which were described in Japanese Official Patent Provisional Publications, showa 63-157879 and 64-68481, because the Na.sub.3 AlF.sub.6 component mingles with a zinc phosphate coating film on an aluminum-based surface, the brine-resistant spraying test and warm brine-resistance of an cationic electrocoating film are bad. Thus, to get a satisfactory quality in a practical use, it is necessary to carry out an after-treatment by a chromium (VI)-containing rinsing solution after the zinc phosphate treatment. The solution containing the chromium (VI) is troublesome in handling and disusing.